Nickel sulfate is industrially used not only in ordinary electroplating but also in electroless nickel plating in producing computer hard discs. In addition, nickel sulfate has been recently used as the nickel material for secondary batteries.
For some of those uses, the amount of impurities such as ammonia, sodium, and others existing in a crude nickel sulfate solution must be reduced as much as possible. To purify nickel sulfate, organic solvent extraction has heretofore been employed. The method of organic solvent extraction using an acid extractant requires a neutralizer both for removing the impurities from the starting crude solution by extracting them into the extractant and for extracting nickel therefrom into the extractant. As the neutralizer, in general, sodium hydroxide is popularly used, as it is easy to handle. Using sodium hydroxide as the neutralizer, however, involves introduction of sodium into the organic phase. As a result, even when the purified nickel sulfate solution is concentrated to give crystals of nickel sulfate, it is extremely difficult to prevent the crystals from being contaminated with sodium.
More concretely, for example, in the conventional method of removing cobalt, calcium and other impurities from a crude nickel sulfate solution through solvent extraction using an acid extractant, an organic acid extractant such as an alkyl phosfonic acid or its ester is normally used. However, sodium of sodium hydroxide that is used as a pH-adjusting neutralizer will naturally remain in the organic phase, resulting in the crystals of nickel sulfate formed from the purified nickel sulfate solution containing sodium. On the other hand, when nickel is extracted from a crude nickel sulfate solution into such an organic acid extractant through solvent extraction, a large amount of sodium will be extracted into the organic phase. Even when the resulting extract is stripped with a mineral acid such as sulfuric acid, most of the sodium moves into the strip solution, resulting in the crystals of nickel sulfate obtained being contaminated with sodium.
Accordingly, in the solvent extraction method using an organic acid extractant for purifying nickel, the nickel-containing organic phase must be scrubbed completely to separate and remove the sodium component. However, this scrubbing is uneconomical and disadvantageous, as it requires a large amount of scrub solution and as the nickel loss in the scrub raffinate is great. In addition, the scrubbing is troublesome since it requires the treatment of the scrub raffinate.